The present invention relates to the study of liquid-liquid interfacial rheology and more particularly to the study of liquid-liquid interfacial rheology through the measurement of electrical impedance across the interface.
The coalescence of liquids in suspension is known to be largely dependent upon the nature of the interfacial properties imparted to the liquid-liquid interface by the accumulation of surfactants and colloids at the interface. The interfacial properties imparted by these suspending agents serve as the foundation for the chemical industry's suspension polymerization businesses. Referring to FIG. 1, there is illustrated a dual interface between an aqueous phase 10 and an organic phase interfacial film 12 (such as oil) as may exist in a coalescence event between two aqueous droplets. A surfactant 14 has been added to the aqueous phase 10 and accumulates at the aqueous/oil interfacial boundary. The surfactant accumulates at the interfacial boundary in order to reduce the interfacial tension between the two different phases 10 and 14.
The addition of the surfactant 14 performs two basic functions in a suspension polymerization process: sizing of droplets and stabilization. The droplet sizing process of a monomer suspension results from a balance between the new surface area being formed from the breakup of droplets and the coalescence of existing droplets. The reduction of interfacial tension between the liquid phases brought about by the accumulation of the surfactant at the interface will, for any given level of work, allow for an increase in surface area (that is, smaller droplets). In order to make efficient use of the work required to produce this increased surface area, the interfacial film with the surfactant should also stabilize the suspension by preventing coalescence. There is, therefore, a need for an analytical tool that will allow this assessment of the surfactant performance characteristics at the interfacial boundary.
There have been numerous attempts made in the prior art to understand the basic mechanisms governing droplet coalescence. The most widely used approach to predict coalescence behavior in dispersed systems has been the use of single droplet coalescence experiments. In these studies, the thin films formed between the droplet and the liquid-liquid interface or between one droplet and another droplet are considered as a model for the emulsion system. For example, Hodgson, T. D. and Lee, J. C., "The Effect of Surfactants on the Coalescence of a Drop at an Interface," J. Colloid Interfacial Sci., 30, (1) 1969, pp. 94-108, described an apparatus and manual technique for forming droplets and measuring coalescence times against a liquid-liquid interface. Scheele, G. E. and Leng, D. E., "An Experimental Study of Factors Which Promote Coalescence of Two Colliding Drops Suspended in Water-I," Chem. Eng. Sci., 26, 1971, pp. 1867-1879, used high speed photography to study colliding droplets and presented a model to predict the coalescence behavior. Flumerfelt et al., "Magnitude and Role of Dynamic Interfacial Effects in Low Tension Flooding," AIChE Symp. Series, V. 78, 1982, pp. 113-126, used a modified spinning drop apparatus for droplet/droplet coalescence experiments as a basis for estimating dilational viscosity for low tension interfacial films.
A significant problem with the experiments described in the prior art is that they do not directly measure any physical properties of the draining film between the coalescing liquids. Rather, from theoretical models and measurements of physical forces (buoyancy, interfacial tension, etc.) and coalescence times, the models are used to calculate the apparent interfacial rheological figures of merit (mainly dilational viscosity and surface shear viscosity). There is therefore a need for an instrumental method that will directly measure a physical property of the draining of the interfacial film during a coalescence event. In addition, all of the prior art techniques for measurement of interfacial rheological properties are manually intensive to set up and operate, do not lend themselves to any form of automated control, and do not provide for accurate repeatability. There is, therefore, a further need for an automated system for measurement of interfacial rheological properties. The present invention is directed towards meeting these needs.